The centrosome is the principal nucleator of the microtubule (MT) cytoskeleton, which is required for cell polarity, vesicle trafficking, and spindle formation and function. While the analogous structure in the yeast S. cerevisiae (the spindle pole body or SPB) is morphologically distinct, a conserved set of Y-tubulin complexes is used to nucleate MT assembly. In this Project we focus on the assembly and regulation of the nucleating machinery using a broad combination of structural approaches (x-ray crystallography, cryoEM single particle reconstruction, cryoEM Tomography) to determine the structures of Y-tubulin complexes in vitro and in situ, and to understand their mechanism of action through quantitative in vitro functional studies and innovative kinetic modeling. Previously we discovered that yeast Y-tubulin small complex (YTUSC) can assemble into rings and obtained a 6.5A cryoEM structure of the rings, explaining the origins of MT 13-fold symmetry and discovering unexpected modes of regulation and assembly. Based on our previous results we propose that there are three phases of regulation: Y-TUSC ring assembly restricted to the spindle pole body by requiring interactions with Spcl 10 or Spc72, ring closure to fully match MT symmetry and, activation of the Y-tubulins for efficient nucleation. The proposed experiments expand upon our previous results with the long-term goal of synthesizing an atomic resolution picture of all the relevant structural and functional interactions between aP- and Y-tubulin complexes, regulatory proteins, and how these complexes are linked to the spindle pole body or centrosome matrix. Specifically we will (i) improve the resolution of our cryoEM reconstruction of yeast YTUSC rings and, collaborate with the Bioinformatics Core to generate a complete pseudo-atomic structure. Structures of yTuSC rings bound to MTs or 1 layer of non-polymerizing yeast ap-tubulin will be determined and compared to structures of in situ capped MT minus ends from cryoEM tomography of yeast SPBs. (ii) We will use a newly developed FRET assay to efficiently measure ring assembly in vitro and determine what domains of Spcl 10 and Spc72 are required for assembly and the role of Spcl 10/72 phosphorylation, (iii) While necessary, assembly into rings is insufficient for potent MT nucleation, with a need for both YTUSC closure to match MT symmetry and an allosteric activation. The role of PTMs or other binding partners in this process will be determined.